[llvm] GlobalISel lane masks merging (PR #73337)

[Petar Avramovic via llvm-commits]

petar-avramovic wrote:

Ping. Updated for comments about code quality. Unchanged for comments about functional changes:
Divergent i1 phis (G_PHIs to be precise) are fully instruction selected into PHIs (all registers created in this pass are lane masks with correct register class and S1 LLT). Check regression tests changes (last two patches in particular) to get better idea what is happening.


https://github.com/llvm/llvm-project/pull/73337